Process for the electrochemical cleavage of lignin at a diamond electrode

ABSTRACT

The invention relates to a process for the electrochemical cleavage of lignin by means of a diamond electrode and also to a process for producing vanillin and derivatives thereof by electrochemical cleavage of lignin in a solution having a pH≦11.

The invention relates to a process for the electrochemical cleavage oflignin by means of a diamond electrode and also to a process forproducing vanillin and derivatives thereof by electrochemical cleavageof lignin in solutions having a pH≦11.

Lignin is a high-molecular-weight aromatic substance which in woodyplants fills up the spaces between the cell membranes and may beconverted to wood. The lignin content of the dried plant material isabout 27 to 33% by weight in conifer wood and 22% by weight in deciduouswood.

Lignin must be seen as a higher-molecular-weight derivative ofphenylpropane. Depending on the wood species, the phenyl ring issubstituted by one to two methoxy groups and the propane units withhydroxyl groups. In coniferous woods, there is predominantly theguaiacyl type, and in deciduous wood, in addition, the syringyl andcoumaryl type. By means of various possibilities of linking, inter alia,lignin and coumarin structures, cyclic ethers and lactones are formed.

Alkali lignin is used in North America as binder for wood- andcellulose-based hardboards, as dispersion medium, for clarification ofsugar solutions, stabilizing asphalt emulsions and also foamstabilization. By far the greatest amount of the alkali lignin, however,is used as energy source for the wood pulp process by combustion of theblack liquors.

Vanillin is widely used in place of the expensive natural vanilla asaroma substance for chocolate, confectionery, liqueurs, bakery productsand other sweet foods and also for the production of vanilla sugar. Thevanillin content of wood which has been processed into wine barrelscontributes to the aromatizing of wine. Smaller amounts are used indeodorants, perfumes and for flavor enhancement of pharmaceuticals andvitamin preparations. Vanillin is also an intermediate in the synthesisof various medicaments such as, for example, L-dopa, methyldopa andpapaverine.

In EP-B 0 245 418, the electrochemical cleavage of lignin for productionof vanillin and its derivatives such as guaiacol and acetovanillone(3-methoxy-4-hydroxyacetophenone) is described. In this case, anaqueous-alkaline solution is employed, wherein heavy metal electrodesare used. The workup proceeds with the use of toxic organohalogensolvents such as chloroform. From (eco)toxicological aspects, this isvery disadvantageous, as is also the use of heavy metal electrodes. Theuse of high sodium hydroxide concentrations, as described in EP 0245418B1, leads to vanillin and its derivatives being present as phenolate orhydroxybenzaldehyde derivatives. The phenolates and hydroxybenzaldehydederivatives of vanillin and of the vanillin derivatives are, however,very sensitive to oxidative processes which should proceed in thealkaline environment. For the workup, therefore, a neutralization mustproceed in advance, so that to obtain vanillin an increased expenditureon workup is necessary.

There is therefore a great requirement for being able to degrade ligninby oxidation in such a manner that workup of the resultant preliminaryproducts requires lower expenditure and therefore is cheaper than theprocesses known hitherto.

This object is achieved by a process for the degradation of ligninhaving a yield of hydroxybenzaldehyde derivatives and/or phenolderivatives higher than 5% by weight, wherein an aqueous solution orsuspension of lignin is electrolyzed at a diamond electrode.

Advantageously in the process according to the invention, an aqueoussolution is employed which has a pH≦11.

Advantageously in the process according to the invention, an aqueous,acidic solution is employed.

Advantageously in the process according to the invention, the diamondelectrode used is a boron-doped diamond electrode.

Advantageously in the process according to the invention, the lignindegradation products are continuously removed from the electrochemicalcell.

Advantageously in the process according to the invention, the lignindegradation products are removed by steam distillation.

Advantageously in the process according to the invention, the lignindegradation products are removed by continuous extraction using anorganic solvent.

Advantageously in the process according to the invention, the lignindegradation products are selected from the group of guaiacol, vanillinand acetovanillone.

The lignin used for the degradation is any lignin known to those skilledin the art. Preference is given to lignin which is present in productswhich are selected from the group of straw, bagasse, black liquor, kraftlignin, lignin sulfonate, organosolv lignin and corresponding residuesfrom the paper industry or fiber production. Particular preference isgiven to the lignin present in kraft lignin and in lignin sulfonate.

The hydroxybenzaldehyde derivatives and/or phenol derivatives which areformed in the degradation of lignin can be obtained at more than 5% byweight using the process according to the invention.

The hydroxybenzaldehyde derivatives and/or phenol derivatives which areformed in the degradation of lignin are selected from the group ofguaiacol, vanillin and acetovanillone. Particular preference is given tovanillin or guaiacol.

The hydroxybenzaldehyde derivatives and/or phenol derivatives which areobtained by the process according to the invention can be continuouslyremoved from the reaction products. Preferably, thesehydroxybenzaldehyde derivatives and/or phenol derivatives arecontinuously removed from the reaction mixture by distillation orextraction. Particular preference is given to steam distillation.

For the electrolysis, the lignin is present in aqueous solution, whereinthe aqueous solution has a pH of ≦11, preferably 9, particularlypreferably is acidic. Very particularly preferably, the pH is 3.Preferably, the pH is adjusted to a pH 3 using readily water-solubleinorganic acids such as hydrochloric acid, sulfuric acid, nitric acid,or organic acid such as para-toluenesulfonic acid or mixtures of variousacids. Particular preference is given to sulfuric acid.

For the electrolysis, use can be made of any electrolysis cells whichare known to those skilled in the art, such as partitioned ornon-partitioned flow cells, capillary gap cells or plate stack cells.Particular preference is given to the non-partitioned flow cell. Toachieve optimum space-time yields, a bipolar arrangement of a pluralityof electrodes is advantageous.

For the process according to the invention, the anode is a diamondelectrode. These diamond electrodes comprise a diamond layer applied toa support material, wherein the support material is selected from thegroup of niobium, silicon, tungsten, titanium, silicon carbide,tantalum, graphite, or ceramic supports such as titanium suboxide.Particularly preferably, the support material is niobium or silicon. Thediamond layer on the support can in addition be doped with furtherelements. Preference is given to boron- or nitrogen-doped diamondelectrodes. Particular preference is given to boron-doped diamondelectrodes.

As cathode material, use can be made of any conventional cathodematerial having a low oxygen overvoltage selected from the group ofRuO_(x)TiO_(x) mixed oxide electrodes (DSA), platinated titanium,platinum, nickel, molybdenum or stainless steel. Preference is given tothe combination of boron-doped diamond cathode with stainless steel ascathode. Particular preference is given to the use of boron- ornitrogen-doped diamond electrodes. Very particular preference is givento boron-doped diamond electrodes.

Use can be made of diamond electrodes which have been produced by theCVD process (chemical vapor deposition). Such electrodes arecommercially available such as, for example, from the manufacturers:Condias, ltzehoe; Diaccon, Furth (Germany) and Adamant Technologies,La-Chaux-de-Fonds (Switzerland). Less expensive diamond electrodes whichhave been produced by the HTHP process (high temperature high pressure:industrial diamond powder is mechanically introduced into the surface ofa support plate) can likewise be used. HTHP-BDD electrodes arecommercially available from pro aqua, Niklasdorf (Austria), theirproperties are described by A. Cieciwa, R. Wüthrich and Ch. Comninellisin Electrochem. Commun. 8 (2006) 375-382.

The temperature for the process according to the invention is between 20and 150° C., preferably in the range from 90 to 120° C.

For the process according to the invention, the current density ispreferably in the range from 5 to 3000 mA/cm², particularly preferablyin the range from 10 to 200 mA/cm². To avoid a coating on theelectrodes, when diamond electrodes are used as anode and/or cathodematerial, the polarity can be changed at short intervals of time. Thepolarity change can proceed in an interval of 30 seconds to 10 minutes,preference is given to an interval of 30 seconds and 2 minutes.

The efficiency of the electrolysis of lignin in aqueous solution atboron-doped diamond electrodes can be increased by the addition ofadditives such as TiO₂. TiO₂ is preferably used in catalytic amounts.

For the electrolysis of lignin, a metal-comprising or metal-free redoxmediator can be added, preference is given to transition metal-freemediators, e.g. nitrosodisulfonates such as Fremy's salt (dipotassiumnitrosodisulfonate).

For the mixing of the cell contents, any mechanical stirrer known tothose skilled in the art can be used, but also other mixing methods canbe used such as the use of an Ultraturrax or ultrasound.

EXAMPLES Example 1

A suspension of 30 g of kraft lignin in an electrolyte comprising 570 gof dilute sulfuric acid (0.1 M) is electrolyzed in a non-partitionedcell which is provided with an electrode stack of boron-doped diamondcathodes (expanded metal, 5×10 cm) and boron-doped diamond anodes(expanded metal, 5×10 cm) at a spacing of 0.5 cm, at a current densityof 80 mA/cm² and a temperature of 30° C. for 1 hour with stirring at9500 rpm. The cell voltage which is established is in the range of 2-6V. The aqueous phase is extracted with methyl tert-butyl ether (MTBE),the solid is filtered off with suction and washed with MTBE. The aqueousphase is repeatedly extracted with MTBE, the organic phases arecombined, dried and the solvent is removed. Gas-chromatographic analysisof the crude organic product gives the following typical composition (GCpercentage areas): 27% guaiacol, 24% vanillin, 24% acetovanillone and25% other compounds. For the GC analysis, the stationary phase used wasa dB1 column from J&W Scientific having a length of 30 m and diameter of0.25 mm and a layer thickness of 1 μm. This column is heated by means ofa temperature program from 80° C. to 250° C. in the course of 5 min in8° C. steps. The carrier gas used is helium having a flow rate of 20 to30 mL/min.

Example 2

Procedure as in example 1 with the following variation: 6 g of kraftlignin, 594 g of dilute sulfuric acid (0.1 M), 3 g of Fremy's salt,electrolysis for 30 minutes at 25° C. Typical composition of organicextracts (GC percentage areas): 37% guaiacol, 23% vanillin, 40%acetovanillone.

1. A process for the degradation of lignin comprising electroyzing anaqueous solution comprising lignin in an electrochemical cell with adiamond electrode to produce a lignin degradation product, wherein thelignin has a yield of hydroxybenzaldehyde derivatives and/or phenolderivatives higher than 5% by weight, and the aqueous solution has aPH≦11.
 2. The process according to claim 1, wherein the aqueous solutionis acidic.
 3. The process according to claim 1, wherein the diamondelectrode is a boron-doped diamond electrode.
 4. The process accordingto claim 1, wherein the lignin degradation product is continuouslyremoved from the electrochemical cell.
 5. The process according to claim1, wherein the lignin degradation products arc product is removed bysteam distillation.
 6. The process according to claim 1, wherein thelignin degradation product is removed by continuous extraction using anorganic solvent.
 7. The process according to claim 1, wherein the lignindegradation product is selected from the group consisting of guaiacol,vanillin and acetovanillone.